LU-7463 osd: Change existence assert into error

LU-7463 osd: Change existence assert into error

In osd_declare_xx(), some objects might not existent,
especially when calling out_handler(). Let's change
these assert into -ENOENT error to avoid panic on
MDS.

Signed-off-by: Di Wang <di.wang@intel.com>
Change-Id: If17a013a6939a1ebe0519406d39e405fd915110f
Reviewed-on: http://review.whamcloud.com/17324
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>